Human diploid cells derived from normal tissue undergo a characteristic pattern of growth in cell culture, ending with a non-replicative phase and, finally, cell death. Models of cellular aging have been developed which suggest analogy between this pattern and behavior of the organism in vivo; hence, the term "senescence". Human diploid fibroblasts (HF) which had been infected in vitro by the oncogenic DNA virus SV40 have been found to overcome this phenomenon. We have found that a clonal isolate of a HF transformed by an origin-defective mutant of SV40 (SVori-) with an intact early region permits isolation of immortalized cells at an unusually high frequency. Analysis of several such immortalized HF from individual cloned transformants will be performed to identify possible common genetic alterations. Further, we propose to study a series of independent HF transformed by origin-defective mutants of SV40 with a temperature-sensitive T antigen (ts A mutants) to assess possible bases for the role of SV40 in the initiation and maintenance of a continuous cell strain.